一玩焰、AsyncTask

AsyncTask是一種輕量級(jí)的異步任務(wù)類逆粹，它可以在線程池中執(zhí)行后臺(tái)任務(wù)募疮，然后把執(zhí)行的進(jìn)度和最終結(jié)果傳遞給主線程并在主線程中更新UI。從實(shí)現(xiàn)上說僻弹，AsyncTask封裝了Thread和Handler阿浓，通過AsyncTask可以更加方便的執(zhí)行后臺(tái)任務(wù)以及在主線程中訪問UI，但是AsyncTask并不適合進(jìn)行特別耗時(shí)的后臺(tái)任務(wù)蹋绽，耗時(shí)任務(wù)建議使用線程池芭毙。

四個(gè)核心方法

1、onPreExecute()卸耘，在主線程中執(zhí)行退敦，在異步任務(wù)執(zhí)行之前，此方法會(huì)被調(diào)用蚣抗，一般可以用于做一些準(zhǔn)備工作侈百。
2、doInBackground(Params ... params)翰铡，在線程池中執(zhí)行钝域，此方法用于執(zhí)行異步任務(wù)，params參數(shù)表示異步任務(wù)的輸入?yún)?shù)锭魔。在此方法中可以通過publishProgress()方法更新任務(wù)進(jìn)度例证，publishProgress方法會(huì)調(diào)用onProgressUpdate方法。該方法需要返回計(jì)算結(jié)果給onPostExecute迷捧。
3织咧、onProgressUpdate(Progress ...values)，在主線程中執(zhí)行漠秋，當(dāng)后臺(tái)任務(wù)的執(zhí)行進(jìn)度發(fā)生改變時(shí)此方法被調(diào)用烦感。
4、onPostExecute(Result result)膛堤，在主線程中執(zhí)行，在異步任務(wù)執(zhí)行之后晌该，此方法會(huì)被調(diào)用肥荔，其中result參數(shù)是后臺(tái)任務(wù)的返回值绿渣，即doInBackground的返回值。
5燕耿、onCancelled()中符，當(dāng)異步任務(wù)被取消時(shí)，該方法被調(diào)用誉帅，此時(shí)onPostExecute則不會(huì)被調(diào)用淀散。

使用限制

1、之前的版本AsyncTask的對(duì)象必須在主線程中創(chuàng)建蚜锨，但最新版本下AsyncTask可以在子線程中創(chuàng)建档插。
2、execute方法必須在UI線程調(diào)用亚再。
3郭膛、不要再程序中直接調(diào)用onPreExecute()、onPostExecute氛悬、doInBackground和onProgressUpdate方法则剃。
4、一個(gè)AsyncTask對(duì)象只能執(zhí)行一次如捅，即只能調(diào)用一次execute方法棍现，否則會(huì)報(bào)運(yùn)行時(shí)異常。
5镜遣、Android3.0以及后續(xù)的版本中己肮，可以通過AsyncTask的executeOnExecutor方法來并行執(zhí)行任務(wù)。

工作原理

    @MainThread
    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        return executeOnExecutor(sDefaultExecutor, params);
    }

    @MainThread
    public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
            Params... params) {
        if (mStatus != Status.PENDING) {
            switch (mStatus) {
                case RUNNING:
                    throw new IllegalStateException("Cannot execute task:"
                            + " the task is already running.");
                case FINISHED:
                    throw new IllegalStateException("Cannot execute task:"
                            + " the task has already been executed "
                            + "(a task can be executed only once)");
            }
        }

        mStatus = Status.RUNNING;

        onPreExecute();

        mWorker.mParams = params;
        exec.execute(mFuture);

        return this;
    }

sDefaultExecutor實(shí)際上是一個(gè)串行的線程池烈涮，一個(gè)進(jìn)程中所有的AsyncTask全部在這個(gè)串行的線程池中排隊(duì)執(zhí)行朴肺。在executeOnExecutor方法中，AsyncTask的onPreExecute方法最先執(zhí)行坚洽，然后線程池開始執(zhí)行戈稿。

    /**
     * An {@link Executor} that executes tasks one at a time in serial
     * order.  This serialization is global to a particular process.
     */
    public static final Executor SERIAL_EXECUTOR = new SerialExecutor();

    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

    private static class SerialExecutor implements Executor {
        final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
        Runnable mActive;

        public synchronized void execute(final Runnable r) {
            mTasks.offer(new Runnable() {
                public void run() {
                    try {
                        r.run();
                    } finally {
                        scheduleNext();
                    }
                }
            });
            if (mActive == null) {
                scheduleNext();
            }
        }

        protected synchronized void scheduleNext() {
            if ((mActive = mTasks.poll()) != null) {
                THREAD_POOL_EXECUTOR.execute(mActive);
            }
        }
    }

AsyncTask的排隊(duì)執(zhí)行過程：首先系統(tǒng)會(huì)把AsyncTask的Params參數(shù)封裝為FutureTask對(duì)象，F(xiàn)utureTask是一個(gè)并發(fā)類讶舰，在這里它充當(dāng)了Runnable的作用鞍盗。接著FutureTask交給SerialExecutor.execute方法處理。在execute方法中跳昼，futureTask首先被插入到任務(wù)隊(duì)列mTask中般甲，如果這個(gè)時(shí)候沒有正在活動(dòng)的任務(wù)，則會(huì)調(diào)用scheduleNext方法執(zhí)行下一個(gè)AsyncTask任務(wù)鹅颊。當(dāng)一個(gè)AsyncTask任務(wù)執(zhí)行完后敷存，AsyncTask會(huì)繼續(xù)執(zhí)行其他任務(wù)直到所有的任務(wù)都被執(zhí)行為止。默認(rèn)情況下，AsyncTask是串行執(zhí)行的锚烦。

AsyncTask中有兩個(gè)線程池（SerialExecutor和THREAD_POOL_EXECUTOR）和一個(gè)Handler（InternalHandler）觅闽，其中線程池SerialExecutor用于任務(wù)的排隊(duì)，而線程池THREAD_POOL_EXECUTOR用于真正地執(zhí)行任務(wù)涮俄，InternalHandler用于將執(zhí)行環(huán)境從線程池切換到主線程蛉拙。

在SerialExecutor中調(diào)用了FutureTask.run()。而在FutureTask.run方法中最后又會(huì)調(diào)用mWorker.call()方法彻亲，因此mWorker.call最終在線程池中執(zhí)行孕锄。

        mWorker = new WorkerRunnable<Params, Result>() {
            public Result call() throws Exception {
                mTaskInvoked.set(true);
                Result result = null;
                try {
                    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                    //noinspection unchecked
                    result = doInBackground(mParams);
                    Binder.flushPendingCommands();
                } catch (Throwable tr) {
                    mCancelled.set(true);
                    throw tr;
                } finally {
                    postResult(result);
                }
                return result;
            }
        };

    private Result postResult(Result result) {
        @SuppressWarnings("unchecked")
        Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }

    ##默認(rèn)情況下handler為InternalHandler
    private static class InternalHandler extends Handler {
        public InternalHandler(Looper looper) {
            super(looper);
        }

        @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
        @Override
        public void handleMessage(Message msg) {
            AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
            switch (msg.what) {
                case MESSAGE_POST_RESULT:
                    // There is only one result
                    result.mTask.finish(result.mData[0]);
                    break;
                case MESSAGE_POST_PROGRESS:
                    result.mTask.onProgressUpdate(result.mData);
                    break;
            }
        }
    }

在mWorker.call()中，首先將mTaskInvoked置為true苞尝，表示當(dāng)前任務(wù)已經(jīng)被調(diào)用過了畸肆，然后執(zhí)行AsyncTask.doInBackground方法，接著將其返回值傳遞給postResult方法野来。postResult方法會(huì)通過sHandler發(fā)送一個(gè)MESSAGE_POST_RESULT消息恼除。sHandler是一個(gè)靜態(tài)的Handler對(duì)象，為了能夠?qū)?zhí)行環(huán)境切換到主線程曼氛，sHandler必須在主線程中創(chuàng)建【其實(shí)按照現(xiàn)在的源碼來看豁辉，我的理解是不需要在主線程中聲明了，看下面代碼】舀患。

    public AsyncTask(@Nullable Looper callbackLooper) {
        mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
            ? getMainHandler()
            : new Handler(callbackLooper);
    }

    private static Handler getMainHandler() {
        synchronized (AsyncTask.class) {
            if (sHandler == null) {
                sHandler = new InternalHandler(Looper.getMainLooper());
            }
            return sHandler;
        }
    }

默認(rèn)情況下AsyncTask持有的mHandler即為sHandler徽级，而目前默認(rèn)給sHandler的looper為主線程Looper，因此就算在子線程中初始化了AsyncTask聊浅，sHandler的handleMessage方法依舊在主線程中執(zhí)行餐抢。
sHandler收到MESSAGE_POST_RESULT消息后調(diào)用AsyncTask.finish()方法。

    private void finish(Result result) {
        if (isCancelled()) {
            onCancelled(result);
        } else {
            onPostExecute(result);
        }
        mStatus = Status.FINISHED;
    }

如果AsyncTask被取消執(zhí)行了低匙，那么回調(diào)mCancelled方法旷痕，否則調(diào)用onPostExecute方法。

自身理解

google的工程師在AsyncTask的構(gòu)造方法中加入了Looper參數(shù)顽冶，意味AsyncTask目前擁有更自由的定制化程度欺抗，我們可以給它傳入子線程下的Looper，同時(shí)當(dāng)前AsyncTask的onPreExecute方法和onPostExecute方法均會(huì)在當(dāng)前線程下回調(diào)而不是默認(rèn)的主線程强重。

二绞呈、HandlerThread

HandlerThread繼承了Thread，它是一種可以使用Handler的Thread间景，在run方法中通過Looper.prepare()創(chuàng)建消息隊(duì)列佃声，并通過Looper.loop()來開啟消息循環(huán)。

public class HandlerThread extends Thread {
    int mPriority;
    int mTid = -1;
    Looper mLooper;
    private @Nullable Handler mHandler;

    public HandlerThread(String name) {
        super(name);
        mPriority = Process.THREAD_PRIORITY_DEFAULT;
    }
    
    /**
     * Constructs a HandlerThread.
     * @param name
     * @param priority The priority to run the thread at. The value supplied must be from 
     * {@link android.os.Process} and not from java.lang.Thread.
     */
    public HandlerThread(String name, int priority) {
        super(name);
        mPriority = priority;
    }

    @Override
    public void run() {
        mTid = Process.myTid();
        Looper.prepare();
        synchronized (this) {
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        Looper.loop();
        mTid = -1;
    }

    public Looper getLooper() {
        if (!isAlive()) {
            return null;
        }
        
        // If the thread has been started, wait until the looper has been created.
        synchronized (this) {
            while (isAlive() && mLooper == null) {
                try {
                    wait();
                } catch (InterruptedException e) {
                }
            }
        }
        return mLooper;
    }

    /**
     * @return a shared {@link Handler} associated with this thread
     * @hide
     */
    @NonNull
    public Handler getThreadHandler() {
        if (mHandler == null) {
            mHandler = new Handler(getLooper());
        }
        return mHandler;
    }

    public boolean quit() {
        Looper looper = getLooper();
        if (looper != null) {
            looper.quit();
            return true;
        }
        return false;
    }

    /**
     * Quits the handler thread's looper safely.
     * <p>
     * Causes the handler thread's looper to terminate as soon as all remaining messages
     * in the message queue that are already due to be delivered have been handled.
     * Pending delayed messages with due times in the future will not be delivered.
     * </p><p>
     * Any attempt to post messages to the queue after the looper is asked to quit will fail.
     * For example, the {@link Handler#sendMessage(Message)} method will return false.
     * </p><p>
     * If the thread has not been started or has finished (that is if
     * {@link #getLooper} returns null), then false is returned.
     * Otherwise the looper is asked to quit and true is returned.
     * </p>
     *
     * @return True if the looper looper has been asked to quit or false if the
     * thread had not yet started running.
     */
    public boolean quitSafely() {
        Looper looper = getLooper();
        if (looper != null) {
            looper.quitSafely();
            return true;
        }
        return false;
    }
}

它與普通Thread有顯著的不同之處倘要，普通Thread主要用于在run方法中執(zhí)行一個(gè)耗時(shí)任務(wù)圾亏，而HandlerThread在內(nèi)部創(chuàng)建了消息隊(duì)列，外部需要通過Handler的消息方式來通知HandlerThread執(zhí)行一個(gè)具體的任務(wù)。
當(dāng)明確不需要再使用HandlerThread時(shí)召嘶，可以通過它的quit或者quitSafely方法來終止線程的執(zhí)行父晶。

三、IntentService

IntentService繼承自Service且自身是一個(gè)抽象類弄跌，可用于執(zhí)行后臺(tái)耗時(shí)的任務(wù)，當(dāng)任務(wù)執(zhí)行后它會(huì)自動(dòng)停止尝苇。IntentService的優(yōu)先級(jí)比單純的線程高很多铛只，適合執(zhí)行一些高優(yōu)先級(jí)的后臺(tái)任務(wù)。

    @Override
    public void onCreate() {
        // TODO: It would be nice to have an option to hold a partial wakelock
        // during processing, and to have a static startService(Context, Intent)
        // method that would launch the service & hand off a wakelock.

        super.onCreate();
        HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
        thread.start();

        mServiceLooper = thread.getLooper();
        mServiceHandler = new ServiceHandler(mServiceLooper);
    }

    @Override
    public int onStartCommand(@Nullable Intent intent, int flags, int startId) {
        onStart(intent, startId);
        return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
    }

    @Override
    public void onStart(@Nullable Intent intent, int startId) {
        Message msg = mServiceHandler.obtainMessage();
        msg.arg1 = startId;
        msg.obj = intent;
        mServiceHandler.sendMessage(msg);
    }

    private final class ServiceHandler extends Handler {
        public ServiceHandler(Looper looper) {
            super(looper);
        }

        @Override
        public void handleMessage(Message msg) {
            onHandleIntent((Intent)msg.obj);
            stopSelf(msg.arg1);
        }
    }

當(dāng)IntentService第一次啟動(dòng)時(shí)糠溜，會(huì)創(chuàng)建一個(gè)HandlerThread淳玩，通過它的Looper創(chuàng)建mServiceHandler。

啟動(dòng)IntentService時(shí)非竿，onStartCommand方法被調(diào)用蜕着，而在onStartCommand中又調(diào)用了onStart方法。

onStart方法中包裝一個(gè)Message對(duì)象红柱，將intent交給mServiceHandler處理承匣，最終由onHandleIntent（）方法處理傳入的Itnent對(duì)象，這里的intent與外界通過startService(Intent intent)方法中傳遞的Intent對(duì)象內(nèi)容一致锤悄，通過這個(gè)Intent即可解析出外界啟動(dòng)IntentService時(shí)傳遞的參數(shù)韧骗，通過參數(shù)區(qū)分具體的后臺(tái)任務(wù)。

當(dāng)onHandleIntent方法執(zhí)行結(jié)束后零聚，IntentService會(huì)通過stopSelf(int startId)方法嘗試停止服務(wù)袍暴，這里之所以采用stopSelf(int startId)而不是stopSelf()停止服務(wù)，因?yàn)閟topSelft()會(huì)立刻停止服務(wù)隶症，而這個(gè)時(shí)候可能還有其他消息未處理政模，stopSelf(int startId)會(huì)等待所有的消息處理完畢后才終止服務(wù)。一般來說蚂会，stopSelf(int startId)在嘗試停止服務(wù)之前會(huì)判斷最近啟動(dòng)服務(wù)的次數(shù)是否和staratId相等淋样，如果相等就立刻停止服務(wù)，不相等則不停止服務(wù)颂龙。

由于IntentService內(nèi)部通過消息的方式向HandlerThread請(qǐng)求執(zhí)行任務(wù)习蓬，Handler中的Looper是順序處理消息的，這就意味著IntentService也是順序執(zhí)行后臺(tái)任務(wù)措嵌，當(dāng)有多個(gè)后臺(tái)任務(wù)同時(shí)純?cè)跁r(shí)躲叼，這些后臺(tái)任務(wù)會(huì)按照外界發(fā)起的順序排隊(duì)執(zhí)行。